The present invention relates to a target to be used for forming a transparent thin oxide film having a high refractive index by direct current (DC) sputtering, and a process for its production, and a method for forming a film having a high refractive index by using such a target.
Optical applications of thin oxide films start from single layer type heat reflecting glasses and antireflection films and extend to various fields including, for example, multi-layer type antireflection coatings, reflection enhancing coatings, interference filters and polarizing films, which are designed to permit lights having certain specific wavelengths to reflect or pass selectively therethrough. Further, a study has been made to insert a transparent electroconductive film or a film of e.g. metal or electroconductive ceramics having various functions such as electroconductivity and heat reflection properties as a part of a multi-layer film to obtain a multi-layer film having a function such as an antistatic, heat reflecting or electromagnetic wave shielding function provided.
The spectral characteristics of a multi-layer film are optically designed by using refractive indexes n and thicknesses of the respective layers, as parameters, and it is common to employ a combination of a high refractive index film and a low refractive index film. To realize excellent optical properties, the larger the difference in the refractive index between the high refractive index film and the low refractive index film, the better. As such a high refractive index film, titanium dioxide (n=2.4) cerium dioxide (n=2.3), zirconium dioxide (n=2.2), niobium pentoxide (n=2.1), tantalum pentoxide (n=2.1) or tungsten trioxide (n=2.0) is, for example, known. Further, as a low refractive index film, silicon dioxide (n=1.46) or magnesium fluoride (n=1.38) is, for example, known.
Such films can be formed, for example, by a vacuum vapor deposition method or a coating method. However, by such a film-forming method, it is difficult to form a uniform film over a substrate having a large area, and when a substrate having a large area, such as a glass for buildings or automobiles, CRT, or a flat display, is required, sputtering is used in many cases. Among various sputtering methods, DC sputtering utilizing direct current discharge is most suitable for forming a film over a large area. When a high refractive index film is to be formed by DC sputtering, it is common at present to employ so-called reactive sputtering wherein a metallic target having electroconductivity is subjected to sputtering in an atmosphere containing oxygen. However, there has been a problem that the film-forming speed of a thin film obtainable by this method is very slow, whereby the productivity is poor, and the cost tends to be high.
To solve such a problem, it has been proposed to use an oxide ceramic (sintered body) as a target. However, oxide ceramic usually has no electroconductivity, whereby DC sputtering has been difficult.
Further, recently, a sputtering target is required to have a complex shape, and a highly efficient planer target having the target thickness partially changed, is required. By a method for obtaining a sintered body by a common sintering method, it is difficult to produce a target having a complex structure or various shapes, and such a target is prepared by a long process including steps of mixing starting materials, sintering, processing and bonding, whereby substantial jigs are required for its production.
In sputtering over a glass sheet with a large area for buildings, the film-forming speed is increased by applying a high power for sputtering to increase the productivity, whereby cooling of the target tends to limit the film-forming speed, and further troubles such as cracking of the target, peeling, etc, are likely to occur.
A new magnetron type rotary cathode is known wherein such drawbacks have been overcome (JP-A-58-500174). This is of a type wherein a magnetic field generating means is provided inside of a cylindrical target, and sputtering is carried out while rotating the target and cooling the target from inside. By the use of such a cylindrical target, a large power per unit area can be applied as compared with a planer type target, whereby film formation at a high speed is said to be possible.
Preparation of a target material on a cylindrical target holder has heretofore been commonly carried out when the target material is a metal or alloy. In the case of a metal target, multi-layer film coatings of e.g. its oxide, nitride, carbide, etc. are formed in various sputtering atmospheres. However, it has had drawbacks that the coating films are likely to be damaged by different types of atmospheres, whereby films having desired compositions can hardly be obtainable, and, in a case of a low melting metal target, the target is likely to undergo melting when the power applied is excessive. Under these circumstances, a ceramic target material has been desired. A method has been proposed in which a ceramic sintered body is formed into a cylindrical shape and bonded to a substrate by means of indium metal. However, the method is difficult and costly.
JP-A-60-181270 proposes a process for producing a ceramic sputtering target by spraying. However, the process has had problems that the sprayed coating can not be made sufficiently thick, as the difference in thermal expansion between the ceramics and the substrate metal is large, and the adhesion tends to deteriorate by thermal shock during its use, thus leading to peeling.
JP-A-62-161945 proposes a process for producing a non-electroconductive ceramic sputtering target made of various oxides by water plasma spraying. This target is a target for radio frequency (RF) sputtering, and the target itself is an insulating material. Further, this target has had drawbacks that, unless some measures such an undercoating is taken, it is likely to undergo cracking or peeling as the temperature rises during sputtering, whereby film formation under a stabilized condition tends to be difficult. Further, there has been a drawback such that the film forming speed is very slow.
It is an object of the present invention to provide an electroconductive sputtering target which can be formed into any desired shape and which is capable of forming a high refractive index film at a high speed by DC sputtering, a process for its production, and a method for forming a high refractive index film using such a target.
The present invention provides a sputtering target comprising a substrate and a target material formed on the substrate, wherein the target material comprises a metal oxide of the chemical formula MOx as the main component, wherein MOx is a metal oxide which is deficient in oxygen as compared with the stoichiometric composition, and M is at least one metal selected from the group consisting of Ti, Nb, Ta, Mo, W, Zr and Hf.
The target of the present invention has electroconductivity and thus is useful for DC sputtering, whereby a uniform, transparent high refractive index film can be formed at a high speed over a large area. The target of the present invention is useful also for RF sputtering.
In a case where M in MOx of the target of the present invention is Nb and/or Ta, x is preferably within a range of 2 less than x less than 2.5. This means that if x is 2.5, the target is electrically insulating, as it is in a completely oxidized state, whereby DC sputtering is not feasible, such been undesirable. On the other hand, if x is 2 or lower, such an oxide is chemically instable and, as such, is not desirable as a target. When NbOx is used, a high film-forming speed can be realized, and when TaOx. is used, it is possible to form a film having a high corrosion resistance and high scratch resistance.
For the same reason as mentioned above, when M in MOx of the target of the present invention is Mo and/or W, x is preferably within a range of 2 less than x less than 3, and when M in MOx in the target of the present invention is at least one metal selected from the group consisting of Ti, Zr and Hf, x is preferably within a range of 1 less than x less than 2. Especially when TiOx is used, it is possible to realize formation of a film having a very high refractive index.
The target of the present invention has electrical conductivity and thus is useful for film formation by means of DC sputtering, whereby a uniform, transparent high refractive index film can be formed at a high speed over a large area. The resistivity at room temperature of the target of the present invention is preferably at most 10 xcexa9cm, more preferably at most 1 xcexa9cm, so that discharge during sputtering can be carried out under a stabilized condition. If the resistivity exceeds 10 xcexa9cm, discharge tends to be hardly stabilized.
For the target of the present invention, a composite oxide MOx employing two or more metals M, may be used so that the target will have the above mentioned characteristics simultaneously.
With the target of the present invention, the properties of the film such as the refractive index, and mechanical and chemical properties, can be changed while maintaining the high speed film formation, by adding an oxide of a metal other than metal M in MOx, as an additive. As such a metal oxide, an oxide of at least one metal selected from the group consisting of Cr, Ce, Y, Si, Al and B, may be mentioned. For example, Cr is capable of imparting corrosion resistance, and Ce is capable of imparting ultraviolet ray-shielding properties.
The target of the present invention can be prepared, for example, as follows.
In a case of a NbOx target, a Nb2O5 powder is subjected to hot-pressing (high temperature and high pressure pressing) for sintering to obtain a target of the present invention. In such a case, the particle size of the powder is preferably from 0.05 to 40 xcexcm. It is important that the atmosphere for the hot-pressing is a non-oxidizing atmosphere, and it is preferred to use argon or nitrogen, since it is thereby easy to adjust the oxygen content in the target. It is also possible to add hydrogen. The hot-pressing conditions are not particularly limited, but the temperature is preferably from 800 to 1,400xc2x0 C., and the pressure is preferably from 50 to 100 kg/cm2.
The present invention also provides a process for producing a sputtering target, which comprises forming an undercoat made of a metal or alloy on a substrate, and forming a ceramic layer as a target material on the undercoat, wherein the ceramic layer as a target material (hereinafter referred to simply as the ceramic layer) is formed by plasma spraying wherein a ceramic powder for spraying (hereinafter referred to simply as the ceramic powder) which is made in a semi-molten state in a high temperature plasma gas in a reducing atmosphere, is transported and deposited onto the undercoat by the plasma gas, and, as the target material, a target material comprising a metal oxide of the chemical formula MOx as the main component, is used, wherein MOx is a metal oxide which is deficient in oxygen as compared with the stoichiometric composition, and M is at least one metal selected from the group consisting of Ti, Nb, Ta, Mo, W, Zr and Hf.
In the present invention, the ceramic powder is made in a semi-molten state by means of a plasma spraying apparatus and deposited on a substrate, so that a ceramic layer for a sputtering target is directly formed.
Accordingly, the process does not require a molding step, a sintering step, a processing step to form a complex structure or shape, or a bonding step. In a case of a complicated compound which is not readily available in the form of a ceramic powder, such a compound may be chemically synthesized or may be prepared by using a solid phase reaction. The ceramic powder may be a pulverized or granulated, and further classified, so that it is adjusted to have a readily flowable particle size suitable for spraying.
The ceramic powder to be used in the present invention can be prepared by the following method. Namely, a TiO2 powder having an average particle size of at most 10 xcexcm and a powder of a metal oxide other than TiO2 having an average particle size of at most 10 xcexcm, are weighed in predetermined amounts and mixed in a wet system for at least 3 hours in a ball mill using a binder such as polyvinyl alcohol (PVA) and water as a dispersing medium, to obtain a slurry, which is then dried by a spray drier to obtain a powder having a particle size of from 10 to 100 xcexcm, preferably from 20 to 100 xcexcm.
In another method, ethanol is used as the above mentioned dispersing medium, and a Nb2O5 powder and a TiO2 powder are mixed together with ethanol in a wet system for at least one hour by means of a ball mill in the same manner as described above, and the mixture is dried by an evaporator and then calcined in an inert atmosphere at a temperature of from 1,000 to 1,200xc2x0 C., followed by classification to obtain a powder having a particle size of from 10 to 100 xcexcm, preferably from 20 to 100 xcexcm. The composition of this powder is reduced by calcination, but is further reduced during the subsequent plasma spraying in a reducing atmosphere.
If the particle size exceeds 100 xcexcm, such a ceramic powder tends to be hardly made in a semi-molten state in a high temperature plasma gas, and if it is smaller than 10 xcexcm, such a powder is likely to be dispersed in the high temperature plasma gas and thus tends to be hardly deposited on the substrate.
For the substrate, various metals or alloys, such as stainless steel, copper or titanium, may be used. Prior to the plasma spraying of a ceramic powder for the target material, it is preferred to roughen the surface of the substrate, for example, by sand blasting by means of abrasive grains made of Al2O3 or SiC in order to improve the adhesion. Otherwise, it is also preferred to process such a substrate surface to form a V-groove, followed by sand blasting by means of abrasive grains made of Al2O3 or SiC, in order to improve the adhesion.
After roughening the substrate surface, an undercoat made of a metal or alloy may be formed in order to reduce the difference in the thermal expansion between the target material to be sprayed and the substrate and to improve the adhesion so as to be durable against peeling by mechanical and thermal impacts.
As such an undercoat, a layer (hereinafter referred to as layer A) having a thermal expansion coefficient intermediate between the substrate and the target material, and/or a layer (hereinafter referred to as layer B) having a thermal expansion coefficient close to the target material, may be used. It is particularly effective to form both layers to have a structure of the substrate/layer A/layer B/ceramic layer. It is preferred to form the undercoat also by plasma spraying.
Even when the undercoat is made solely by layer A or layer B, the adhesive force of the ceramic layer to the substrate can be improved, since the metal or alloy is not brittle and has high elasticity. The thermal expansion coefficient of layer B is most suitably within a range of xc2x12xc3x9710xe2x88x926/xc2x0 C. of the thermal expansion coefficient of the ceramic layer.
As the material for the undercoat, an electroconductive powder of e.g. Mo, Ti, Ni, Nb, Ta, W, Nixe2x80x94Al, Nixe2x80x94Cr, Nixe2x80x94Crxe2x80x94Al, Nixe2x80x94Crxe2x80x94Alxe2x80x94Y or Nixe2x80x94Coxe2x80x94Crxe2x80x94Alxe2x80x94Y may be employed. The thickness of the undercoat is preferably from 30 to 100 xcexcm.
It is necessary to change the material for the undercoat depending upon the thermal expansion coefficient of the ceramic layer. The thermal expansion coefficient of e.g. copper or stainless steel which is useful as a substrate, is from 17xc3x9710xe2x88x926 to 18xc3x9710xe2x88x926/xc2x0 C., and the thermal expansion coefficient of titanium is 8.8xc3x9710xe2x88x926/xc2x0 C.
For example, for the ceramic layer (the thermal expansion coefficient: 6xc3x9710xe2x88x926 to 9xc3x9710xe2x88x926/xc2x0 C.) in the present invention, the preferred thermal expansion coefficient of the undercoat layer A is from 12xc3x9710xe2x88x926 to 15xc3x9710xe2x88x926/xc2x0 C., and as such a material, Ni, Nixe2x80x94Al, Nixe2x80x94Cr, Nixe2x80x94Crxe2x80x94Al, Nixe2x80x94Crxe2x80x94Alxe2x80x94Y or Nixe2x80x94Coxe2x80x94Crxe2x80x94Alxe2x80x94Y, may, for example, be mentioned.
Further, the preferred thermal expansion coefficient of the undercoat layer B is from 4xc3x9710xe2x88x926 to 11xc3x9710xe2x88x926/xc2x0 C., and as such a material, Mo, Nb, Ta, W or Ti, may, for example, be mentioned.
Further, the adhesion can be further improved by providing an undercoat layer having the composition gradually changed from a material having a thermal expansion coefficient close to the target material to a material having a thermal expansion coefficient close to the substrate, selected among such undercoat materials. Further, when the substrate is made of titanium, the undercoat may be made solely of layer B, since the thermal expansion coefficient is close thereto.
On such an undercoat, a ceramic powder which is made in a semi-molten state in a high temperature plasma gas, preferably a high temperature plasma gas such as Ar or Ar+H2, in a reducing atmosphere, is transported and deposited onto the undercoat by such a gas, to form a ceramic layer which serves as a target material. In this manner, the oxide ceramic powder is reduced, and a ceramic layer comprising MOx as the main component, is obtained.
By forming the undercoat, the difference in thermal expansion between the ceramic layer and the substrate can be reduced, whereby a ceramic layer which is free from peeling even with a thickness as thick as from 2 to 10 mm, can be formed.
Also at the time of forming the undercoat, it is preferred to form it by plasma spraying in a high temperature plasma gas, preferably in a high temperature plasma gas in a reducing atmosphere, for the same reason as described above.
Further, as the plasma spraying method, water plasma spraying is more effective. This water plasma spraying is a method wherein a high pressure water stream supplied to a torch firstly forms a cylindrical eddy current at the cylindrical section, and in this state, a voltage is applied between a carbon cathode and an iron rotary anode to let direct current arcs form to evaporate and recompose water at the inner surface of the eddy current to form a plasma state thereby to generate plasma arcs continuously, and such plasma arcs are constricted by the revolving cylindrical water current to increase the energy density and eject a stabilized high temperature high speed plasma jet flame from a nozzle by rapid thermal expansion of the plasma. This spraying method provides a high energy density as compared with a gas plasma density, and a large amount of the starting material powder can thereby be sprayed all at once, whereby the target forming speed is high, and the economical efficiency is high. Further, it is thereby possible to readily form a thick film.
However, as compared with plasma spraying by a reducing gas, the reducing power is weak. Therefore, to obtain a state of MOx, it is better to employ a material which is reduced at the stage of the starting material powder.
The present invention further provides a method for forming a high refractive index film employing the above described target.
A uniform transparent film can be formed at a high speed, when sputtering is carried out by using the target of the present invention in an argon atmosphere or in a mixed atmosphere of argon and small amount of O2 under a pressure of from 1xc3x9710xe2x88x923 to 1xc3x9710xe2x88x922 Torr. In a case where a metal target is employed, a hysteresis phenomenon occurs which is a non-continuous change in the film forming speed or the discharge current or voltage due to change of the oxygen partial pressure. However, when the target of the present invention is employed, such a hysteresis phenomenon will not occur, and control of the film forming speed during the film formation will be very easy.
When a metal target is used for forming a metal oxide film, the film forming speed or the sputtering voltage changes abruptly and non-continuously in a hysteresis fashion due to a change of the oxygen gas partial pressure before or after the change from an absorbing film to a transparent film having the stoichiometrical composition, or before or after the change from a transparent film to an absorbing film. Accordingly, to obtain a transparent film constantly, it is necessary to introduce oxygen gas substantially excessively relative to the metal atoms.
Whereas, the target of the present invention is composed of an oxide, and is slightly deficient in oxygen as compared with the stoichiometric composition. Accordingly, film formation of a transparent metal oxide film can be carried out simply by supplementing the oxygen slightly deficient as compared with the stoichiometric composition. Besides, when the target of the present invention is employed, no change like the above mentioned hysteresis phenomenon will take place, whereby the amount of the oxygen gas to be supplied can be minimized to the required minimum or reduced to a level close to the required minimum. Thus, deposition of excess oxygen atoms on the target surface, which is believed to cause deterioration of the film forming speed, can be reduced, and the film forming speed can be increased.
When a target is produced by a spraying method as in the present invention, the oxide powder is made in a molten state and then quenched and solidified so that the sprayed material will be laminated on the substrate. At that time, crystal alignment will form in the sprayed material, since there is a difference in the crystal growth rate of crystal faces. Namely, in the face at which the surface density is low and the growth rate is high, crystallizes quickly in the direction along the substrate, and crystal alignment will necessarily form, so that the face at which the surface density is high and the growth rate is low, becomes the sputtering surface.
On the other hand, it is believed that the higher the surface density, the better the sputtering efficiency, and the higher the sputtering speed. Accordingly, this crystal alignment is believed to be one of the factors for the high film forming speed by the present invention. Further, sputtering speed is believed to be increased by numerous defective layers formed in or between grains at the time of quenching and solidification, which are susceptible to etching as compared with normal tissues.
Further, with a sputtering target thus prepared, thermal conductivity from the ceramic layer to the substrate and further to the cathode electrode, is good, and the ceramic layer is firmly bonded to the substrate. Accordingly, even when a high sputtering power is applied to increase the film forming speed, cooling can sufficiently be carried out, and peeling or cracking of the ceramic layer due to abrupt heat shock will not take place, and a large electric power per unit area can be applied.
Further, even when an erosion zone of the ceramic layer became thin, such a zone can readily be regenerated to the initial state by plasma spraying a ceramic powder of the same material to such a portion which became thin. Further, it is easy to provide a distribution in thickness of the ceramic layer depending upon any desired position, and it is thereby possible to control the thickness distribution of a thin film to be formed by providing a temperature distribution or a distribution in strength of the magnetic field at the target surface.
Further, when a cylindrical substrate is employed, the entire surface will be the erosion zone of the target, whereby there is a merit that the utilization efficiency of the target is high as compared with the planer type.